Plasma arc cutting systems are widely used for cutting metallic materials and can be automated for automatically cutting a metallic workpiece. In general, a plasma arc cutting system includes a plasma arc torch, an associated power supply, a remote high-frequency (RHF) console, a gas supply, a positioning apparatus, a cutting table, a torch height control, and an associated computerized numeric controller. FIG. 1 shows an example of a plasma arc cutting system.
In operation, a user places a workpiece on the cutting table and mounts the plasma arc torch on the positioning apparatus or motion control mount to provide relative motion between the tip of the torch and the workpiece to direct the plasma arc along a processing path. The user provides a start command to the computerized number controller (CNC) to initiate the cutting process. The CNC accurately directs motion of the torch and/or the cutting table to enable the workpiece to be cut to a desired pattern. The CNC is in communication with the positioning apparatus. The positioning apparatus uses signals to the CNC to direct the torch along a desired cut path. Position information is returned from the positioning apparatus to the CNC to allow the CNC to operate interactively with the positioning apparatus to obtain an accurate cut path.
The power supply provides the electrical current necessary to generate the plasma arc. The power supply has one or more dc power modules to produce a constant current for the torch. Typically, the current can be set to discreet values. The power supply has a microprocessor, which regulates essentially all plasma system functions, including start sequence, CNC interface functions, gas and cut parameters, and shut off sequences. For example, the microprocessor can ramp-up or ramp-down the electrical current. The main on and off switch of the power supply can be controlled locally or remotely by the CNC. The power supply also houses a cooling system for cooling the torch.
The gas console controls flow of plasma and shield gases to the torch. The gas console houses solenoid valves, flow meters, pressure gauges, and switches used for plasma and shield gas flow control. The flow meters are used to set the preflow rates and cut flow rates for the plasma and shield gases. The gas console also has a multi-inlet gas supply area where the required plasma and shield gases can be connected. A toggle switch can be used to select the plasma gases. The plasma and shield gases are monitored by gas pressure gages.
The RHF console houses a high frequency starting circuit that is used to fire the torch. The RHF console also houses a cathode manifold used to interface power and coolant leads between the power supply and the torch. The power and coolant leads and a pilot arc lead make up a shielded torch lead set which connects with the torch. In addition, gas lines are also supplied to the torch to supply gas.
Plasma arc torch systems use a positional apparatus such as a motion driving system to control the motion of the torch. The motion driving system can include a torch height control that controls the height of the torch relative to the workpiece. It can also position the tip of the torch proximate the workpiece, move the torch about the surface of the workpiece, and control torch angles, such as bevel.
In bevel and robotic applications, the torch movement can involve 6-axis maneuverability. For example, when a bevel or chamfer cut is made, the edge of the workpiece is not perpendicular. Instead, the edge of the workpiece is cut at an angle, for example at a 45 degree angle. Cutting a workpiece at an angle requires the torch to rotate or swivel on an axis that is not used for traditional, straight edge plasma torch cutting. These additional axes of maneuverability in bevel and robotic applications can cause stress or wear of the lead set due to the excess twisting and rotating in these applications. This can lead to early lead set failure.
The plasma arc torch generally includes a torch body, an electrode mounted within the body, passages for cooling fluid and cut and shield gases, a swirl ring to control the fluid flow patters, a nozzle with a central exit orifice, and electrical connections. A shield can also be provided around the nozzle to protect the nozzle and to provide a shield gas flow to the area proximate the plasma arc. Gases applied to the torch can be non-reactive (e.g., argon or nitrogen) or reactive (e.g., oxygen or air).
The plasma arc cutting system described above can be used in cutting applications that involve 6-axis maneuverability, including bevel, chamfer, and robotic applications. This type of maneuverability results in extreme twisting and bending of the lead set, which causes the lead sets to fail prematurely. Prior solutions require complex programming to determine the number of rotations a torch has made during a cutting process, so that the system can pause and “unwind” the lead set at regular intervals in the hopes of preventing excessive premature failure of the lead set. Failure of the lead set and pausing to “unwind” the lead set results in unwanted and costly downtime of the system and is only minimally effective.